The present invention relates to a voltage-to-current converter which can be used as, e.g. a control circuit for controlling an electronic volume, and which outputs a current linearly varying in proportion to an input voltage.
FIG. 4 shows a conventional voltage-to-current converter. The converter comprises a PNP transistor Q1 of emitter follower type and an NPN transistor Q2 having a base connected to the emitter (i.e. output terminal) of the transistor Q1. To be more specific, the transistor Q1 having its base connected to an input terminal 41, its collector to a ground terminal 44 and its emitter to a power terminal 42 through a constant current source I1. Furthermore, the emitter of the transistor Q1 is connected to the base of the transistor Q2. The collector of the transistor Q2 is connected to an output terminal 43, and the emitter of the transistor Q2 is connected to the ground terminal 44 through a resistor R1.
In the above conventional converter, an input voltage Vin is applied to the input terminal 41, and an output current Iout is supplied from the output terminal 43. The current Iout flows forward, from the output terminal 43 to the collector of the transistor Q2. The current Iout is expressed as follows: EQU Iout=(Vin+VBEQ1-VBEQ2)/R1
where VBEQ1 is the voltage between the base and the emitter of the transistor Q1, and VBEQ2 is the voltage between the base and the emitter of the transistor Q2.
If VBEQ1 is equal to VBEQ2, the output current Iout varies in proportion to the input voltage Vin.
In the conventional converter, VBEQ1 and VBEQ2 are given by: EQU VBEQ1=VT 1n.times.(I1/IS) EQU VBEQ2=VT 1n.times.(Iout/IS)
where VT is kT/q or a thermoelectromotive force constant, and IS is a saturation current. The saturation currents of the transistors provided in integrated circuits are equal if the integrated circuits have been made in the same process. The transistor Q1 is biased with the constant current I1, and the transistor Q2 is biased with the output current Iout. Voltages VBEQ1 and VBEQ2 are therefore not equal. The difference between VBEQ1 and VBEQ2 is not expressed by a linear function. This means that the variation .DELTA. Iout of the output current is not proportional to the variation .DELTA. Vin of the input voltage.
Jpn. Pat. Appln. KOKAI Publication No. 5-259755 discloses a voltage-to-current converter. However, the converter needs a large number of current mirror circuits, and thus has a complicated structure.